The present invention generally relates to semiconductor devices. More specifically, the present invention relates to semiconductor light-emitting devices and processes for producing same.
It is known that semiconductor light emitting devices can be fabricated by forming a low temperature buffer layer overall on a sapphire substrate, forming an n-side contact layer made from GaN doped with Si thereon, and stacking an n-side cladding layer made from GaN doped with Si, an active layer made from InGaN doped with Si, a p-side cladding layer made from AlGaN doped with Mg, and a p-side contact layer made from GaN doped with Mg thereon. As commercial products of semiconductor light emitting devices having such a structure, light emitting diodes and semiconductor lasers for emitting light of blue and green having a wavelength of 450 nm to 530 nm have been fabricated on a large scale.
With respect to growing gallium nitride (GaN), a sapphire substrate has been often used; however, in this case, dislocations may be contained in the grown crystal at a high density because of mismatching in lattice between the sapphire substrate and the gallium nitride to be grown thereon. From this viewpoint, a technique for forming a low temperature buffer layer on a substrate is effective to suppress defects caused in the crystal to be grown on the substrate. Further, a method of reducing crystal defects by usual epitaxial growth in combination with epitaxial lateral overgrowth (ELO) has been disclosed in Japanese Patent Laid-open No. Hei 10-312971.
Japanese Patent Laid-open No. Hei 10-312971 regarding a method of fabricating a semiconductor light emitting device describes that through-dislocations propagated in the direction perpendicular to a substrate principal plane is deflected in the lateral direction by a facet structure formed in a growth region during fabrication of the device, so that it is possible to block the propagation of the through-dislocation and hence to reduce crystal defects.
A light emitting system including a plurality of semiconductor light emitting devices in the form of light emitting diodes or semiconductor lasers is usable for an image display unit by using, as each of pixels arrayed in a matrix, a combination of light emitting diodes or semiconductor lasers of blue, green, and red, and independently driving the pixels; and is also usable for a white light emitting unit or an illumination unit by making the light emitting devices of blue, green, and red simultaneously emit light of blue, green, and red. In particular, since a light emitting device using a nitride semiconductor has a band gap energy ranging from about 1.9 eV to about 6.2 eV, it can realize a full-color display only by using one material. For this reason, a multi-color light emitting device using a nitride semiconductor has been actively studied. It is to be noted that the term “nitride” used herein means a compound which contains one or more of B, Al, Ga, In, and Ta as group III elements and N as a group V element, and which may contain impurities in an amount of 1% or less of the total amount or 1×1020 cm3 or less.
A technique of forming a multi-color light emitting device on the same substrate has been known, wherein a plurality of regions for emitting light of respective colors, which include active layers having different band gap energies corresponding to different emission wavelengths, are stacked, and a common electrode on the substrate side is provided while electrodes on the other side are individually provided on the light emission regions. In another known multi-color light emitting device, the regions for emitting light of respective colors are stepwise formed on the substrate for easy extraction of electrodes therefrom. The multi-color light emitting device of this type in which a plurality of layers including a pn-junction are stacked has a possibility that the light emission regions in the same device act just as a thyristor, and to prevent such operation similar to that of a thyristor, a multi-color light emitting device, in which grooves are formed between one and another of the stepwise light emission regions for isolating the light emission regions from each other, has been disclosed, for example, in Japanese Patent Laid-open No. Hei 9-162444.
Further, a light emitting device disclosed in Japanese Patent Laid-open No. Hei 9-92881 is configured such that, to realize multi-color light emission, an InGaN layer is formed on an alumina substrate via an AlN buffer layer, wherein a portion of the InGaN layer is doped with Al to form a blue light emission region, another portion of the InGaN layer is doped with P to form a red light emission region, and a non-doped portion of the InGaN layer is taken as a green light emission region.
The above-described techniques, however, have the following problems. Known epitaxial lateral overgrowth techniques and known crystal growth methods characterized by forming a facet structure in a growth region are advantageous in that since the propagation of through-dislocations can be deflected by a facet structure portion or the like, crystal defects can be significantly reduced. However, to form a light emission region including an active layer after epitaxial lateral overgrowth or formation of the facet structure, the epitaxial lateral overgrowth is further performed or the facet structure is buried so as to obtain a flat plane on which the light emission region is to be formed, with a result that the number of processing steps is increased and a time required for fabricating the device is prolonged.
Known multi-color light emitting devices are disadvantageous in that since the processing steps become complicated, it fails to form the light emitting device at a high accuracy, and since the crystallinity is degraded, it fails to provide good light emission characteristic. For the multi-color light emitting device in which grooves are formed between one and another of the stepwise light emission regions for isolating the light emission regions from each other, anisotropic etching must be repeated by several times for isolating the light emission regions including active layers from each other. This causes problems that since the crystallinity of each of the substrate and the semiconductor layer may be degraded by dry etching, it is difficult to sustain desirable crystallinity, and that since etching is repeated by several times, the number of steps required for mask alignment and etching is increased.
For the multi-color light emitting device in which impurities are selectively doped in the single active layer formed on the substrate, since a margin must be provided for forming an opening portion in the mask layer, a sufficient distance must be set between one and another of the different light emission regions, particularly, in the case of previously estimating a fabrication error, so that it is difficult to form a micro-side light emitting device, and further, the number of steps is increased by selective doping.